The satellite-earth optical communication technology adopts optical communication links between satellites and the earth by means of laser beams as a carrier. As compared with the microwave communication technology commonly used at present, the satellite-earth optical communication technology has advantages such as high communication capacity, small system size, light weight, high security, little electromagnetic interference, and broad band. Thus, many countries have put great efforts to researches on the satellite-earth optical communication technology. Among those countries, Japan firstly succeeded in a satellite-earth optical communication test based on an ETS-VI system in July 1995. The test proved the feasibility of the satellite-earth laser communications. US JPL (Jet Propulsion Lab) developed an OCD (Optical Communications Demonstrator) with a data transfer rate up to 250 Mbps. The STRV2 satellite-earth laser communication project funded commonly by US BMDO (Ballistic Missile Defense Organization) and Space and Missile Defense Command planned to establish an optical communication link, with a distance of 2000 km and a data transfer rate of 1 Gpbs, between a LEO (Low Earth Orbit) satellite and a stationary ground station. AREMIS, a GEO satellite developed by ESA (European Space Agency), was launched in 2000, and has an optical link which can implement communications between this satellite and a ground station located on Canary Island.
The ATP (Acquisition, Tracking, Pointing) technology is a key one to be improved in the field of the satellite-earth optical communications. An ATP system is composed of a tilt tracker, a control unit and a drive unit. In operation of the system, the tilt tracker is able to provide an amount of tilt in a wavefront of a target to the control unit in real time. The control unit calculates an amount of voltage to be loaded to the drive unit based on the tilt amount of the wavefront. The drive unit can rotate by a certain angle in a certain direction due to the voltage, so that an entrance pupil of the system can be aligned with the object in moving. (See Jiangtao XIA, Design of an Optical Collimation and Automatic Tracking System, Electronics Optics & Control, 2009, 16(5), 74-77.)
In an ATP system for the satellite-earth optical communications, optical transmission must pass through the atmosphere which is a random channel. Due to low-order aberrations caused by atmospheric turbulence and also the movement of the satellite, a light spot arriving at the system may present a wide range of random jitters (see Xiaofeng LI & Yu HU, “Effect of Background Light and Atmospheric Turbulence to Spots Received in Space-to-Ground Laser Communications,” Wireless Optical Communications, 2004(10), 22-24). Further, in the satellite-earth optical communications, laser signals from the satellite have limited transmitting power due to loading capacity limitations of space crafts, and will have most energy thereof scattered and absorbed by the atmosphere after a long travel in the atmosphere (see Shuhua LIU, “Solution Designs, Analyses on Key Techniques, and Channel Simulations in Space-to-Ground Communications System,” a dissertation for Master Degree in University of Electronic Science and Technology of China, 2002). To ensure a fluent satellite-earth optical communication link, the tilt tracker in the ATP system, which provides the tilt amount in the wavefront of the target, must meet requirements, such as a wide dynamic range, a high detection precision, a high sensitivity, a high frame frequency, and the like.
The tilt tracker generally comprises an imaging lens, a photoelectric converter, and a wavefront tilt handling device. An optical signal from the target is converged by the imaging lens to be projected onto a photosensitive surface of the photoelectric converter to form a target spot. When the target has the tilt amount of its wavefront changed, the target spot moves on the photosensitive surface of the photoelectric converter. As a result, a distribution of optical energy on the photosensitive surface is changed. At this time, the wavefront tilt handling device can calculate the position of a centroid of the target spot based on an electric signal outputted from the photoelectric converter, and thus derive the tilt amount in the wavefront of the target. Currently, the photoelectric converter is generally implemented by a CCD camera, a CMOS camera, or a multi-anode PMT. However, the CCD camera is limited in applications of high frame frequency detections due to its disadvantages such as low readout frame frequency, and the CMOS camera is limited in applications of weak light detections due to its disadvantages such as large noise and low photosensitivity. The multi-anode PMT is composed of multiple PMTs (PhotoMultiplier Tubes) integrated into a single package based on a certain spatial design, with signals outputted at different pins corresponding to different spatial locations of optical cathodes. Thus, the multi-anode PMT can function as a multi-pixel spot-centroid detector.
However, due to manufacture processes, there is no shield provided at anodes of the multi-anode PMT. As a result, when the multi-anode PMT operates in a photon counting state, generation of a photoelectric pulse at one pixel is accompanied by generation of a smaller electrical pulse at another pixel, due to inter-electrode capacitance coupling. If the smaller pulse caused by such crosstalk becomes greater than a threshold level, it will be deemed by a backend circuit as a photoelectric pulse, resulting in a dummy photon count. The dummy photon count significantly impacts the accuracy of the outputted signals and thus the accuracy of the spot-centroid detection.
In order to ensure the requirements on the tilt tracker in the ATP system adopted in the satellite-earth optical communications, such as high detection precision, high sensitivity, high frame frequency, and the like, it becomes an important research object to provide a solution to reduce the impacts of the inter-pixel crosstalk in the multi-pixel spot-centroid detector on the spot-centroid detection accuracy.